Idea is really cool and fits the theme perfectly, controls are simple and easy to understand, art, music and overall presentation are perfect.
The idea of knocking off surfers from above and hijacking the board unfolds with jumping-off-water mechanic and 2d-aiming controls. But it seems like the dolfin turns way too slow for the pace. Same for controlling the board - I barely could do anything when on it. Also, I would add animation for knocked people just to show that they're still alive (otherwise it seems too dark for the atmosphere)
Nice game for the jam!